Threshold voltage is one of important characteristics that affect the performance of a transistor. The threshold voltage of a transistor is often affected by its well doping concentration. To increase the threshold voltage of a transistor, the well doping concentration may be increased, and vice versa.
The increase in the well doping concentration, however, faces challenges. For example, Fin Field-Effect Transistors (FinFETs) have semiconductor fins as their channels. With the increasing down scaling of integrated circuits, the semiconductor fins become increasingly narrower. When high threshold voltages are needed for the FinFETs, higher doses of impurities are implanted into the semiconductor fins to increase the well doping concentrations. Due to the very narrow fins, in the subsequent thermal processes, however, most of the implanted impurities may be diffused away, and the well doping impurity that is left in the final structure is becoming an increasingly smaller portion. As a result, heavier doses of impurities are needed.
Increasing the already high implantation doses, however, results in high threshold voltage variations. The out-diffusion of the impurity from a narrow fin is affected by various factors, and has high variations. From wafer to wafer, and even from FinFET to FinFET on the same wafer, the out-diffusion may vary significantly. This results in the well doping concentration of the FinFETs to vary also. The threshold voltages of the resulting FinFETs thus have a high variation.